Many buildings contain raised access floor systems, which allow power and data communications wiring to be located in the floor plenum below the access floor. This cavity may also be used as an underfloor air delivery system for the supply of conditioned air to the occupancy space above. Air is typically delivered from the air delivery system to the occupancy space via one or more floor diffusers, and air may be removed from the space from a high level. Known diffusers, however, may not provide sufficient airflow patterns, air pattern adjustment and airflow rate adjustment to maximise user comfort and minimise air conditioning energy consumption.